1. Field of the Invention
This invention relates to a brake timing control method and apparatus therefor and, more particularly, to a method and apparatus useful in connection with a variable speed press machine for bringing the press slide to stop at its top dead center in every stroke thereof, regardless of the speed of operation of the press machine or the name of strokes of the press slide per minute which are customarily represented by revolutions per minute (r.p.m.).
2. Description of the Prior Art
In machining articles with a press machine such as including a press slide driven through a crank shaft by a variable speed motor, it is often necessary in view of safety, easy finished article removal and other considerations to provide such a brake timing control as enabling the press slide to positively stop at its top dead center in every stroke thereof. The prime factor to consider in such brake timing control is the slip angle through which the crank shaft as well as the press slide continues to rotate after braking is initiated. Such slip angle is caused by inertia forces of the moving parts such as the press slide, the crank shaft, etc., and its degree varies in proportion to the speed of rotation of the crank shaft or the number of strokes of the press slide. It is therefore apparent that braking on the press machine should be initiated when the crank shaft reaches an angular position prior to the top dead center by a slip angle according to the number of strokes of the press slide concerned.
One of conventional brake timing controls has been accomplished by employing several rotary cams mounted for coordinated or synchronized rotation with the crank shaft to actuate respective rotary cam switches. One of the rotary cam switches is selectively actuated by the associated rotary cam in accordance with the speed of rotation of the crank shaft or the number of strokes of the press slide for initiating braking on the press machine when the crank shaft reaches a predetermined angular position. However, such convertional brake timing control often fails to stop the moving press slide at its top dead center.
As an example, a press machine will be considered which is operable at strokes ranging from 100 r.p.m. to 220 r.p.m. and exhibits a slide angle of 60 degrees at 100 r.p.m. and a slide angle of 180 degrees at 220 r.p.m. If only one rotary cam is provided so as to actuate its associated rotary cam switch for initiating braking on the press machine when the crank shaft rotates at 180 derees, the crank shift will stop at its top dead center (180+180=360 degrees) at the 220 r.p.m., while it will stop at an angular position of 180+60=240 degrees at the 100 r.p.m.. That is, such brake timing control using one rotary cam will cause a maximum angular position dispersion of 360-240=60 degrees. If three rotary cams are provided, one of which is selected to actuate its associated rotary cam switch in accordance with the number of strokes of the press slide, the dispersion will be reduced to one-third the dispersion found in the above brake timing control using only one rotary cam and rotary cam switch pair. The dispersion can be further reduced by providing an increased number of rotary cam and rotary cam switch pairs, but this will result in a complex and expensive arrangement.